1. Field of the Invention
The present invention relates generally to methods for the treatment or prevention of UV-induced immunosuppression. More particularly, the invention relates to the treatment of UV-induced immunosuppression of the contact hypersensitivity type (CHS), through the application of liposome-encapsulated DNA repair enzymes.
2. Description of the Related Art
Wavelengths of UV radiation in the middle, or UV-B (280-320 nm), range can impair a variety of immune responses in humans and laboratory animals both locally, within UV-irradiated skin, and systemically, at distant sites (1). Exposure of mice to UV-B radiation interferes with the rejection of UV-induced skin cancers and the induction of delayed and contact hypersensitivity (DHS, CHS) responses initiated at unirradiated sites; these forms of immune suppression are associated with the induction of antigen-specific suppressor T lymphocytes (2). How UV-B radiation exerts its systemic, immunosuppressive effects is a question of considerable interest, both for understanding the regulatory pathways governing these immune responses and for assessing the potential effects of UV-B radiation on human health. The DHS response is particularly important in this regard because this T-lymphocyte-mediated immune reaction is responsible for protection against many chronic infectious diseases.
Current experimental evidence implicates soluble substances derived from UV-irradiated keratinocytes as the probable mediators of UV-induced systemic suppression of DHS and CHS responses (3-5). However, the initial photobiological reaction responsible for triggering the cascade of events leading to activation of the suppressor pathway of the immune response remains controversial. Based on an in vivo action spectrum for systemic suppression of CHS in the mouse, it has been proposed that urocanic acid, a deamination product of histidine, present in the stratum corneum, is the photoreceptor for this form of UV-induced immunosuppression (6). Several subsequent studies supported this hypothesis (7-9) by demonstrating that the cis isomer of urocanic acid, which is formed upon UV irradiation of the native molecule, has immunosuppressive activity in mice.
On the other hand, studies using the South American opossum Monodelphis domestica implicated DNA damage as the initiating event in UV-induced suppression of CHS, at least in these distant relatives of eutherian mammals (10). Cells from these marsupials have the ability to repair UV-induced pyrimidine dimers in DNA by means of a photoreactivating enzyme, which binds to DNA, forming a complex that absorbs energy in he visible wavelength range. The absorbed energy splits the dimers, thereby restoring the DNA to its original configuration. In these studies, UV-induced suppression of CHS was prevented by exposing the opossums to photoreactivating light immediately after exposure to UV radiation (10). These results implicated DNA damage as the trigger for UV-induced suppression of CHS in marsupials; however, its role in immune suppression in placental mammals remained open to question. Because cells in the skin of adult mice do not have a photoreactivation mechanism for repairing pyrimidine dimers (11), this approach could not be used to determine the role of DNA damage in UV-induced immunological effects in the mouse.
The present inventors have devised an alternative approach for examining the role of DNA damage in the initiation of UV-induced immunosuppression in mice by using liposomes containing an excision repair enzyme, T4 endonuclease V, which is specific for pyrimidine dimers (12). When topically applied to murine skin, T4N5 liposomes penetrate cells of the epidermis (13, unpublished results). The multilamellar lipid vesicles destabilize at low pH, thereby delivering the endonuclease intracellularly and increasing the rate of repair of pyrimidine dimers in DNA (14,15).
The preparation and use of liposome encapsulated DNA repair enzymes in the treatment of certain diseases and conditions related to UV irradiation, including xeroderma pigmentosum and certain cancers, has been described (see, e.g., Yarosh, U.S. Pat. No. 5,077,211 and PCT application publication number WO 90/00598, published Jan. 25, 1990, both incorporated herein by reference). However, previously there has been no indication that the damage to DNA caused by UV irradiation was related to immunosuppression. Thus, there was no reason to believe that therapies directed to DNA repair would have an effect upon reversing or ameliorating UV-induced immunosuppression. The present invention surprisingly provides a basis for this conclusion.
In particular, the present invention presents studies wherein the inventors have analyzed the role of pyrimidine dimers, induced in DNA by UV irradiation in vivo, in initiating systemic suppression of the DHS and CHS responses. The results of these studies demonstrate the usefulness of liposome-encapsulated DNA repair enzymes in the treatment of both systemic immunosuppression of the DHS as well as the CHS response. Accordingly, the present invention expands our previous understanding of the causal relationship between UV irradiation, DNA repair and cancer, to demonstrate a relationship between DNA repair and reversal of UV-mediated immunosuppression.